Photocatalysis

The photocatalytic performance of MgFe2O4 nanoparticles and macroporus CaFe2O4 was investigated using different self-built reactor setups.

3.8.8.1 Hydrogen evolution

Hydrogen evolution experiments using visible light were attempted at 20 °C using a Newport Sol1A solar simulator equipped with a Xe lamp operated at 145 W, proving irradiation from the top. The UV part of the Xe spectrum was removed by an AM1.5G filter that was placed in the light path. For irradiation of the samples with UV-light, a mid-pressure Hg lamp operated at 500 W in an inner-irradiation geometry was used and the temperature was kept at 10 °C. All gas evolution experiments were performed under magnetic stirring in a sealed reactor-detector array, flushed with Ar as carrier gas. Gas detection was performed using either a Shimadzu GC 2014 gas chromatograph equipped with a TCD detector, or a Hiden HPR-20 Q/C quadrupole mass spectrometer. For photodeposition of co-catalysts, up to 1 wt.-% Rh or Pt were added during the experiment, in the form of Na3RhCl6 and H2PtCl6, respectively. For overall water splitting tests, 0.1 wt.-% of RhCrOx nanoparticles were deposited previously to the experiment adapting a procedure by Zhao et al.^[168]

3.8.8.2 Degradation of organic compounds

Degradation of nitrobenzene was attempted at 20 °C, using a suspension of 100 mg of photocatalyst in a 10^-4 M ethanolic solution of nitrobenzene. Irradiation was performed from the top using a Newport Sol1A solar simulator equipped with Xe lamp operated at 145 W, both with and without AM1.5G filter in the beam path. The degradation of nitrobenzene was monitored by UV-Vis spectroscopy.

Degradation of rhodamine B (RhB) was attempted at 20 °C, using a suspension of 100 mg of photocatalyst in a 10^-5 M aqueous solution of RhB. Irradiation was performed from the top using a Newport Sol1A solar simulator equipped with Xe lamp operated at 145 W, both with and without AM1.5G filter in the beam path. The degradation of RhB was monitored by UV-Vis spectroscopy.

Degradation of 2,6-dichloroindophenol was attempted at 0 °C, using a suspension of 50 mg of photocatalyst in an aqueous solution containing 5∙10^-5 M 2,6-dichloroindophenol and 10^-2 M KNO3. In one case 2.3 w% of Pt were deposited prior to the degradation experiment by adapting a procedure by Baumanis et al.^[169] Irradiation was performed from the top using a solar simulator equipped with Xe lamp operated at 300 W. A longpass filter (λ>360 nm) placed in the light path, to remove UV-light.

The degradation of 2,6-dichloroindophenol was monitored by UV-Vis spectroscopy.

3.8 Characterization techniques and instrumentation

3.8.9 (Photo-)electrochemistry

For all electrochemical measurements, ferrite photoelectrodes were mounted in a PTFE cell with a 1 cm² quartz window, containing a solution of 0.1 M Na2SO4 as electrolyte (Figure 28). For chopped-light voltammetry (CLV) and incident photon to current efficiency (IPCE) spectroscopy, 0.015 M of H2O2

were added to the electrolyte. Measurements were performed with a Zahner Zennium potentiostat using a three-electrode configuration with the photoelectrode acting as working electrode, a Pt wire as counter electrode and Ag/AgCl in 3M NaCl as reference electrode.

Figure 28: Schematic depiction of the PTFE cell that was used in all photoelectrochemical measurements.

3.8.9.1 Mott-Schottky analysis

Mott-Schottky analysis is an electrochemical method for the determination of flat band potentials and donor densities of semiconductor electrodes. The flat band potential is defined as the external potential that must be applied to cancel out the band bending that occurs at the semiconductor-liquid interface, due to fermi level equilibration. For highly n-doped and p-doped semiconductors, the flat band potential is located close to the CMB or VBM respectively. Therefore Mott-Schottky analysis provides a means to determine the potential of one of the band edges. When the band gap of the semiconductor is known (for example from optical spectroscopy) the energy of both band edges can be approximated. To construct a Mott Schottky plot, the inverse square capacitance of the semiconductor-electrolyte interface is recorded, while the external potential applied to the electrode is varied. The electrochemical system is hereby commonly described by a simple capacitor equivalent circuit and the capacitance is determined from the imaginary part of the electrochemical impedance spectrum. The course of the Mott-Schottky plot is described by the Mott-Schottky equation (Equation 12), where C is the capacitance, ε is the relative permittivity of the sample, ε0 is the permittivity of vacuum, A represents the area of the electrode that is in contact with the electrolyte, ND is the donor density and Vfb is the flat band potential.

3.8 Characterization techniques and instrumentation

Consequentially the donor density can be calculated from the slope of the linear region of the Mott-Schottky plot, while a positive or negative slope indicates n-type and p-type semiconducting behavior, respectively. The flat band potential can be determined from the intersection of the extrapolated linear slope with the abscissa. In the case of more complex samples like nanostructured ones, Mott-Schottky analysis comes with a large measurement uncertainty, since the assumed capacitor model is usually too simple to describe such systems. The assumption of an improper equivalent circuit may even result in a completely wrong interpretation of the obtained data.^[170] Nevertheless, the technique is routinely applied for the determination of the band potentials of both bulk-, and nanostructured photocatalysts. In this work, Mott-Schottky analysis was performed on ferrite photoelectrodes that were prepared by spray-coating.

3.8.9.2 Chopped-light linear sweep voltammetry and incident photon to current efficiency For a chopped light voltammetry, the external bias applied on a photoelectrode is continuously altered over a given potential range, while the current generated from a photoelectrochemical half-reaction on the semiconductor-electrolyte interface, is measured. During the potential sweep, irradiation of the sample with light (of appropriate wavelength for an excitation of valence electrons into the conduction band) is periodically switched off and on. The difference of the photocurrent (generated by the exited charge carries) from the dark current is a measure for the performance of the photoelectrode.

Depending on the semiconducting nature of the photoelectrode, the detected photocurrent is either positive (n-type) or negative (p-type). Information about band positions and overpotentials for the investigated electrochemical reaction can be derived from the onset potential of the photocurrent. For CLV measurements in this work, 1 cm² of the ferrite photoelectrode were illuminated with a white light LED emitting in a range of 400–800 nm.

In incident-photon to current efficiency (IPCE) spectroscopy, the photocurrent at a fixed external bias is put into relation to the known photon flux of a tunable light source and can therefore be used to investigate, whether the absorption characteristics of a material can be efficiently exploited for photocurrent generation. For IPCE spectroscopy, illumination was performed with a commercial Zahner TLS03 LED array. The irradiated area on the photoelectrode was 1 cm².